Drilling holes in metal is fundamental to manufacturing products from metal and can be a significant part of the total manufacturing effort. Hole size, required accuracy, and drilling speed are important manufacturing considerations that determine the types of cutting tools and machinery used to drill holes. Cutting tool wear and hole accuracy present related problems that are particularly severe in high speed drilling. These and other related problems currently arise during use of both conventional and insert drills.
Conventional drills have cutting edges formed at a cutting end that extend from near the central axis to the outer radius of the drill and helical chip channels extending along the drill from the cutting edges. Chips cut by conventional drills are the width of the cutting edge. Wide metal chips can coil within the chip channel preventing chip evacuation from the cutting end. The resulting chip accumulation can cause heating of the cutting end of the drill causing the cutting edge to become dull or break. Further, excessive heat can harden the workpiece being drilled. Conventional drills are usually long enough to drill deep holes making the drill relatively flexible. Fast drilling can cause a conventional drill to vibrate, causing the drilled hole to have unacceptable accuracy and a poor surface finish.
Insert drills offer significant advantages over conventional drills for high speed manufacturing. An insert drill has replaceable cutting tip inserts mounted in pockets formed at the cutting end of the insert drill. Each insert has a cutting edge positioned at the cutting end of the insert drill. A cutting tip insert may have multiple cutting edges and may be mounted in an insert pocket in various orientations that position different cutting edges at the cutting end of the drill. Such inserts are referred to as indexable inserts. Insert drills conventionally are two flute drills. Typical two flute insert drills are shown in U.S. Pat. Nos. 4,230,429 and 4,558,975.
Cutting tip inserts are mounted at different radial locations at the cutting end of an insert drill so that each cutting tip insert cuts across a radial region between the drill axis and the outer radius of the drill. Cutting tip inserts overlap to provide continuous cutting from the drill axis to the radial extent of the insert drill. The length of the cutting edge of a cutting tip insert determines the width of the chips that are cut by that single cutting tip insert. A two flute insert drill having two cutting tip inserts cuts chips that are about one half the width of the chips cut by a conventional drill of the same size. Small chips diminish the possibility of clogging the chip channel. The size of a cutting tip insert is limited to a maximum dimension to prevent vibration and breakage of the cutting tip insert. Larger diameter insert drills may have two or more inserts separated from each other along a single flute. In that case, cutting tip inserts in the other flute are positioned to extend across the gaps between inserts in the first flute and radially overlap them to provide cutting along the entire radius of the insert drill.
The insert pockets of insert drills must position the cutting tip insert to achieve optimal cutting and be sized to support the cutting tip insert. Insert pockets are conventionally machined in the insert faces at the cutting end of the insert drill by a cutter extending generally perpendicular to the insert face. This method requires unobstructed access to the entire insert pocket. The configuration of two flute insert drills easily accommodates this method of machining insert pockets.
While two flute insert drills offer significant improvements over conventional drills for high speed manufacturing, large loads must be applied to the drill to achieve high speed drilling. Large loads can cause vibration of the drill, heating of the cutting edges, and wear and breakage of the cutting tip inserts. Further, large drill loads can cause a slug of metal ahead of the drill at a through-surface of the workpiece to be pushed out as the drill is forced through the through-surface. The slug is approximately the size of the hole and may be ejected from the workpiece of high speed creating a hazard to persons near the workpiece. Further, a large burr is created as the slug is pushed out of the workpiece.
The design and use of insert drills is constrained by the need to minimize problems resulting from drill vibration and heating. To minimize vibration and maintain accuracy of the drilled hole, very stiff insert drills are typically limited to lengths that can drill holes less than 2.5 times the diameter of the drill. A second machine operation is often required to assure accuracy of the hole dimensions and an acceptable surface finish. Insert drills are often provided with cooling passages opening near the cutting end of the insert drill to allow coolant to be pumped into the chip channel adjacent the cutting tip to enhance transport of chips away from the cutting edge and cool both the insert drill and the workpiece.
The use of three flute insert drills has been suggested to diminish the severity of the problems associated with the two flute insert drill. U.S. Pat. No. 4,230,429 contains a drawing of a three flute insert drill. However, there is no suggestion for a method of manufacturing the three flute insert drill. As is readily apparent, the insert pocket of a three flute insert drill closest to the center of the drill, cannot be manufactured by the method used to machine insert pockets in two flute insert drills. To provide unobstructed access to the innermost insert pocket along an axis perpendicular to the insert face, a substantial amount of the flute opposite the innermost cutting tip would have to be removed. The flute would be unacceptably weakened, diminishing the support of the cutting tip insert mounted to that flute.
The need therefore exists for a method of manufacturing a three flute insert drill which does not require removing a significant amount of material from flutes of the drill at the cutting end of the drill. In addition, there is a need for an insert drill that will allow high speed drilling of holes without requiring large loads. Further, there is a need for an insert drill that cuts metal during high speed drilling without requiring large loads to be applied to the drill.